Conventionally, a DCCT (DC Current Transformer) of a magnetic modulation type, for example, is known as a beam current meter.
However, in the conventional DCCT of a magnetic modulation type, the lower limit for measuring and electric current is in an order of several μA and there has been a problem that the meter could not measure a faint beam current of about several nA.
To solve such a problem, as a beam current meter capable of measuring the faint beam current of about several nA, a beam current meter using a SQUID (Superconducting Quantum Interference Device), which is used for measuring a magnetic field generated by a brain or a heart, and a magnetic shield that consists of a super-conductive body operating at a liquid helium temperature, has been developed by GSI (Gesellschaft fur Schwerionenforschung) of Germany, old Institute for Nuclear Study of Tokyo University, or Research Center for Nuclear Physics of Osaka University (refer to Non-patent document 1, Non-patent document 2, Non-patent document 3, Non-patent document 4 and Non-patent document 5 to be described later).
It is to be noted that the beam current meter capable of measuring a faint beam current of about several nA has as 1000 times higher sensitivity than that of a conventional beam current meter for measuring a beam current of an order of several μA. In a comparison of specific magnetic fields, since earth magnetism is 10−5 T, a cerebric magnetic field is 10−15 T, and a point 20 cm apart from the center of a magnetic field created by a beam of 1 nA is 10−15 T, the beam current meter capable of measuring a faint beam current of about several nA must measure a very faint magnetic field.
Herein, FIG. 1 and FIG. 2 show structural schematic constitutions of a conventional beam current meter using the above-described SQUID and the magnetic shield that consists of the super-conductive body operating at a liquid helium temperature. Specifically, FIG. 1 and FIG. 2 show only mechanical and structural constitutions contributing to the understanding of the present invention, and illustration of various electrical connecting states, electrical conductive states and means for detecting temperature or the like are omitted. It is to be noted that FIG. 1 is a sectional view taken along A-A line of FIG. 2, and FIG. 2 is a sectional view taken along B-B line of FIG. 1.
Further, in the description of this specification and the accompanying drawings, the same or corresponding constitutions and contents are shown by using the same reference characters, duplicate explanation for the constitutions and functions will be omitted.
In FIG. 1 and FIG. 2, reference character 1 designates a beam current sensor composed of a super-conductive body, reference character 2 designates a magnetic shield composed of the super-conductive body, reference character 3 designates a SQUID, reference character 4 designates a cooling medium tank, reference character 5 designates a vacuum vessel, reference character 6 designates an upper flange, reference character 7 designates a beam duct, reference character 8 designates a trestle, reference character 9 designates liquid helium being the cooling medium, reference character 10 designates a vacuum area in the vacuum vessel 5, and reference character 11 designates an atmospheric air area outside the vacuum vessel 5.
More specifically, the vacuum vessel 5 is constituted in such a manner that a side of the upper surface 5a is closed by the upper flange 6, the through holes 5c that constitute the beam ducts 7 are severally formed on facing positions on a circumferential wall surface, and a side of the bottom surface 5b is supported on the trestle 8.
In the beam current meter, a beam is made incident from one of the through holes 5c, which constitutes one of the beam ducts 7, and is output from the other through hole 5c that constitutes the other beam duct 7.
Then, the cylindrical beam current sensor 1 is installed in the vacuum vessel 5 such that the beam that is made incident into the vacuum vessel 5 passes through its inner diameter, and the SQUID 3 is installed on the upper surface side of the beam current sensor 1. Further, the cylindrical magnetic shield 2 composed of a super-conductive body is installed surrounding the outer diameter side of the beam current sensor 1 so as to allow the SQUID 3 to position between the shield and the beam current sensor 1.
The above-described beam current sensor 1, the SQUID 3 and the magnetic shield 2 are arranged in the doughnut-shaped cooling medium tank 4, and each of the above-described constituent members is arranged so as to allow the beam to pass a hollow region in the inner diameter side of the doughnut-shaped cooling medium tank 4.
It is to be noted that liquid helium being the cooling medium is filled in the cooling medium tank 4, and the beam current sensor 1, the SQUID 3 and the magnetic shield 2 arranged in the cooling medium tank 4 are cooled down to liquid helium temperature.
In the above-described constitution, the inside of the vacuum vessel 5 is maintained at 1×10−4 Pa by a vacuum unit (not shown), and the beam is allowed to pass to the beam current meter. Specifically, the beam is allowed to pass so as to be made incident from one through hole 5c that constitutes one beam duct 7, and is output from the other through hole 5c that constitutes the other beam duct 7, and the beam current of the beam is measured.
Although the constitutions and functions of the beam current sensor 1 and the SQUID 3 and a measuring principle of the beam current meter using them are widely known technology, they will be briefly explained referring to FIG. 3 for the purpose of easier understanding of the present invention.
FIG. 3 shows the schematic constitution perspective conceptual exemplary view of the beam current sensor 1 where the SQUID 3 is attached to a bridge unit (described later) of the beam current sensor 1, and FIG. 4 shows a partially enlarged conceptual exemplary view of the schematic constitution of an area indicated by arrow A of FIG. 3.
The beam current sensor 1 is composed of a super-conductive body where a super-conductive material is formed on a substrate made of an insulator of a cylindrical shape. However, the super-conductive material is not formed circumferentially (in a headband state) on a surface of the outer diameter side at the central position in an axis direction of its circumferential wall surface, except for a linear part area being a bridge unit, and an insulator having a linear shape, that is, the substrate is exposed circumferentially (in a headband state) except for the linear bridge unit (refer to FIG. 4).
In short, in the beam current sensor 1, the linear insulator is formed circumferentially (in a headband state) on the surface of the outer diameter side of the circumferential wall of the cylindrical super-conductive body while only the bridge unit being the linear part region as it is. The above-described insulator is circumferentially arranged at the central position in the axis direction of the beam current sensor 1. Further, the SQUID 3 is arranged on the above-described bridge unit.
When the beam passes through a space of the inner diameter side of the beam current sensor 1, a shielding current flows on the surface of super-conductive body due to the Meissner effect. The shielding current flows only in the bridge unit, and a magnetic field in an azimuth angle is formed by the passage of the current.
Specifically, forming the bridge unit on the surface of the cylindrical super-conductive body makes it possible to efficiently concentrate the shielding current. By placing the SQUID 3 on the bridge unit, a magnetic field created by a current is measured and the measured magnetic field is converted into a current value, and the beam current can be measured non-destructively and highly accurately.
To measure the magnetic field formed in the azimuth angle direction on the bridge unit with good SN ratio, it is preferable to use a SQUID gradiometer as the SQUID 3.
The reason is that input coils for detecting a magnetic field are on the right and left of the SQUID gradiometer as shown in FIG. 4, in the case where an external noise magnetic flux is about to enter the right and left input coils, the external noise magnetic flux is completely cancelled if it has a common mode noise magnetic field of completely the same size and direction of the external noise magnetic flux and on the other hand, a magnetic field formed on the bridge unit the passage of a beam has a negative phase magnetic field of the same size as described above but opposite directions, and it can be detected with twice the sensitivity of a SQUID magnetometer being a one input coil type that is generally used.
By employing the SQUID gradiometer as the SQUID 3, the external noise magnetic field can be remarkably reduced, and it has become possible to remarkably improve the sensitivity limitation of a conventional DCCT of a magnetic modulation type by applying such technology of superconductivity.
Meanwhile, the conventional beam current sensor explained in above, it has been difficult to detect a magnetic field formed by a faint beam current of about 1 nA because a magnetic field formed in the air was detected by the SQUID to measure the beam current value, and has had a problem that it was difficult to measure a faint beam current value of about 1 nA.
Non-patent document 1: “A Cryodevice for induction monitoring of DC electron or ion beams with nano-ampere resolution”, K. Grohamann, et al., Superconducting Quantum Interference Devices and Their Application, 1977, p. 311
Non-patent document 2: “SQUID based beam current meter”, IEEE Trans. On Magnetics, Vol. MAG-21, No. 2, 1985, p. 997
Non-patent document 3: “A cryogenic current comparator for the absolute measurement of nA beams”, AIP Conf. Proc. 451 (Beam Instrumentation Workshop), 1998, p. 163
Non-patent document 4: “Design and performance of an HTS current comparator for charged particle-beam measurements”, L. Hao et al., IEEE Trans. On Appl. Supercond. (ASC2000), Vol. 11, No. 1, 2001-3, p. 635
Non-patent document 5: “High sensitivity measurement of beam current in storage rings”, Tetsumi Tanabe, Kei Shinada, Bulletin of The Physical Society of Japan, Vol. 54, No. 1, 1999, p. 34